1. Field of the Invention
The present invention relates to a video/audio decoding technique in conformity with the ATSC (Advanced Television Systems Committee) standard, and more particularly to phase compensation processing in a signal receiving system.
2. Description of Related Art
Recently, there is known ATSC standard as a digital broadcasting standard in the United States. FIG. 1 is a schematic diagram of a system according to the ATSC standard. As illustrated, the transmission system applies the encoding processing onto the video original signal to be transmitted and the audio original signal to be transmitted independently of each other, multiplexes them by a predetermined method and then supplies them to the transmission path encoding and modulating unit. As shown in FIG. 2A, the transmission path encoding and modulating unit applies the data randomizing, the Reed-Solomon encoding, the convolutional data interleaving and the Trellis encoding onto the inputted multiplexed signal. Then, the transmission path encoding and modulating unit adds the sync and further adds the pilot signal, and then applies the equalization and VSB-modulation. Finally, the transmission path encoding and modulating unit transmits the VSB-modulated signal as the RF signal.
On the other hand, the receiving system basically performs the reverse processing to the transmitting system. Namely, the transmission path decoding and demodulating unit shown in FIG. 2B receives the transmitted RF signal and detects the sync and the pilot signal. Then, the transmission path decoding and demodulating unit eliminates the NTSC signal, if necessary, and applies the waveform equalization, the phase processing, the Trellis decoding, the de-interleaving, the Reed-Solomon decoding and the de-randomizing to recover the multiplexed signal of the video signal and the audio signal. Then, the transmission path decoding and demodulating unit de-multiplexes to obtain the video signal and the audio signal, and decodes them to reproduce the video original signal and the audio original signal.
As shown in FIG. 2B, in the receiving system in conformity with the ATSC standard, the output signal from the NTSC rejection filter is subjected to the waveform equalization and then is subjected to the phase compensation by the phase tracker. The waveform equalizer stage is constituted by a feedforward filter and a feedback loop including a slicer and a feedback filter for obtaining estimated transmission symbols. The phase tracker estimates phase error from the signal after the waveform equalization, and compensates for the estimated phase error. However, in such a system, since the waveform equalization stage includes non-linear processing by the feedback loop having the slicer, the estimation capability of the phase error by the phase tracker provided downstream of the waveform equalization stage is deteriorated if large echo exists.
It is an object of the present invention to provide a waveform equalization and phase compensation circuit in a digital signal receiving system, which is capable of performing accurate phase compensation even if the transmission path includes large echo.
According to one aspect of the present invention, there is provided a signal processing circuit for a digital signal receiving system, including: an equalization unit for performing waveform-equalization of a received signal, the equalization unit including a backward equalizer constituted by a feedback loop; and a phase compensation unit for performing phase compensation of the received signal, wherein the phase compensation unit is arranged upstream of the backward equalizer.
In the signal processing circuit thus configured, since the phase compensation unit is arranged upstream of the backward equalizer, it is possible to avoid the deterioration of the phase compensation ability of the phase compensation unit by the influence from the echo in the transmission path.
Preferably, the equalization unit may further include a forward equalizer, the phase compensation unit being arranged between the forward equalizer and the backward equalizer. Thus, the equalization can be appropriately performed with avoiding the deterioration of the phase compensation ability.
In a preferred embodiment, the backward equalizer may include a slicer for estimating transmission symbols; a feedback equalizer for receiving the estimated transmission symbol from the slicer; and an adder for adding the output of the phase compensation unit to the output of the feedback equalizer and for supplying the result of the addition to the slicer. By this, the deterioration of the phase compensation ability is avoided by arranging the compensation unit upstream of the slicer which includes non-linear processing.
In another preferred embodiment, the backward equalizer may include a Trellis decoder for estimating transmission symbols; a feedback equalizer for receiving the estimated transmission symbol from the Trellis decoder; and an adder for adding the output of the phase compensation unit to the output of the feedback equalizer and for supplying the result of the addition to the Trellis decoder. By this, highly reliable waveform equalization can be achieved by the accurate transmission symbol estimation by the Trellis decoder, with avoiding the deterioration of the deterioration of the phase compensation ability. The Trellis decoder may output Trellis decoding result corresponding to the transmission symbols. Thus, a single Trellis decoder can perform both the transmission symbol estimation for the waveform equalization and the Trellis decoding.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to the preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.